You should never need to pay again if trying to re-download the HappyCow app from the same app store (Apple, Google, or Amazon) if you are using the same app store ID/email address you used when originally purchased the app.
However, if you are trying to download from a different store, that won't work as Apple, Google, and Amazon are big rivals and their stores are unrelated, so the app doesn’t carry over to re-download.
If you have trouble with something else, please open a ticket and we’ll see if we can help.